Allow sharing of calendars

Please, allow sharing of added calendars. This can de offered with a share icon of three nodes connected with two vertices. At least sharing via copy paste buffer is important.

Benefit is that calendar urls which need to be reused for adding a variation of that url makes it the user easier. Calendar urls are usually not the most easiest to enter. Also sharing them with others via e-mail, Telegram, etc. would then be possible. Note that sharing with copy paste buffer only holds the url. Sharing via other ways may also hold the title of the calendar.